The present invention relates to a carrying unit for carrying, for example, a sheet-like workpiece while holding it as a carried object.
The carrying unit for carrying a workpiece such as a semiconductor wafer while holding it as a carried object from a workpiece supporting position to a processing position includes a type which carries the carried object by sucking it by vacuum and a type which carries the carried object while holding it in a non-contact condition without keeping the carried object in contact with a carrying head.
As the former type, vacuum suction type is available and as the latter type, a non-contact carrying unit is available, which as described in Patent document 1(Japanese Patent Laid-open Publication No. 10-181879), has a carrying head in which a concave portion is formed opposing the carried object while a nozzle forming a ring-like slit for discharging positive pressure air along an air guide face formed in the concave portion is mounted in the concave portion. In this type of the carrying unit, air layer is formed on the front end face of the carrying head with air discharged from the slit, so as to create negative pressure condition in front of the front end face of the carrying head, whereby sucking the carried object and holding the carried object via the air layer. Suction holding force for the carried object depends on flow velocity and flow amount of airflow along the front end face of the carrying head.
In case of the above-described non-contact type carrying unit, if the gap of the slit is constant, it is necessary to change the flow velocity and flow amount of air by means of a pressure control valve or a flow amount control valve in order to change the suction holding force. In order to attract a carried object to the carrying head by creating negative pressure condition in front of the front end face of the carrying head, a large amount of air needs to be discharged from the slit. After the carried object is held by the carrying head via the air layer, the carried object can be held by a smaller amount of discharged air than that discharged for attracting the carried object. Such a non-contact type carrying unit is so constructed that a specified amount of air is always discharged from the slit and therefore, a large amount of air needs to be discharged in carrying process.
Positive pressure air flows outward in the diameter direction along the front end face of the carrying head from the ring-like slit, so that negative pressure condition is created. If the carried object is a member easy to deform like an ultra thin workpiece, the carried object is deformed, which is a problem to be solved.
Further, when a carried object, after carried through suction process and carrying processes, is loaded on a predetermined place, supply of positive pressure air to the slit is stopped to allow the carried object to drop naturally by its own weight. If it takes long to drop the carried object, productivity of such a product goes down, which is another problem to be solved.